1. Field of the Invention
This invention relates generally to circuits and, more specifically, the present invention relates to circuits including series stacked capacitors.
2. Background Information
One function of a power converter is to convert rectified alternating current (AC) power into a regulated direct current (DC) output. FIG. 1 shows elements included at an input 103 to a power converter 101. Diode bridge BR1 105 rectifies AC input. Series stacked capacitors C1 107 and C2 109 are coupled across diode bridge BR1 105 to smooth the output voltage of diode bridge BR1 105. Series stacked capacitors are common in power supplies that can be configured to operate using multiple different input voltages such as for example either 115VAC or 230VAC. As shown, switch SW1 111 is coupled between diode bridge BR1 105 and the connection point 113 between series stacked capacitors C1 107 and C2 109. When operating for example at 230VAC, switch SW1 111 is opened. When operating for example at 115VAC, switch SW1 111 is closed.
Resistors R1 115 and R2 117 are coupled across series stacked capacitors C1 107 and C2 109 as shown to maintain roughly equal voltages across C1 107 and C2 109 and provide the necessary bleed current needed to balance the voltage across series stacked capacitors C1 107 and C2 109 when the supply is configured for 230VAC input. During 115VAC operation, the two series stacked capacitors C1 107 and C2 109 function as part of an input voltage doubler circuit. When configured for 230VAC, however, the two series stacked capacitors C1 107 and C2 109 have no DC connection to a center point voltage other than that which is provided by resistors R1 115 and R2 117. Without resistors R1 115 and R2 117, the center point voltage of the two series stacked capacitors C1 107 and C2 109 can deviate from the ideal ½ DC input due to capacitor leakage current, which can cause one capacitor to have more voltage stress than the other capacitor. In fact, it is possible that one of the capacitors can be over-voltage stressed and become damaged.
Resistors R1 115 and R2 117 therefore provide a solution to the balance problem by providing bleed current. The resistance values of R1 115 and R2 117 must be low enough to establish a bleed current that is several times higher than the worst case leakage current imbalance between the series stacked capacitors C1 107 and C2 109 in order to be effective. This requires the resistors to dissipate much more power than the actual power dissipated as a result the difference current between the two capacitors. Consequently, resistors R1 115 and R2 117 result in significant input power consumption with respect to many standby or output no-load requirements of a power supply converter coupled to receive the rectified AC power.